It is well known that a chemical mediator released by antigen-antibody reaction, e.g., histamine, plays an important role in inducing allergy. Typical known agents having an inhibitory activity on release of chemical mediators include sodium cromoglycate as disclosed in British Pat. No. 1,144,906. This compound, however, is not effective in oral administration and is generally administered by inhalation or by other topical applications as an aqueous solution thereof. Apparently, the inhalation has difficulty in administering the compound to infants or children and, moreover, can not be administered to patients sensitive to irritation caused by powder, and the topical application of aqueous solution is limited only to nasal cavity. Thus, development of excellent anti-allergic agents which can be administered orally has long been desired.
As a result of extensive studies on compounds having an anti-allergic activity, the present inventors found that the triazolopyrimidine derivatives of the present invention represented by the formula (I) below exhibit an excellent anti-allergic activity.
The condensed ring included in the compounds of this invention, i.e., thiazolo[3,2-a][1,2,3]triazolo[4,5-d]pyrimidine and [1,3,4]thiadiazolo[3,2-a][1,2,3]triazolo[4,5-d]pyrimidine, are novel condensed ring which have been first synthesized by the present inventors, and the compounds represented by the formula (I) having various substituents thereon are novel compounds.
The compounds represented by the formula (I) of this invention have the following characteristics:
(a) These compounds exhibit an inhibitory activity on release of chemical mediators induced by the antigen-antibody reaction, not only release of histamine which has been well known as a chemical mediator, but also release of a so-called "slow reacting substance of anaphylaxis" (SRS-A) which has recently been known to play an important role as a chemical mediator in the attack of asthma.
(b) These compounds exhibit such inhibitory activity in oral administration.